The present invention relates to a process for the removal and recovery of one or more phenolic compounds from an aqueous fluid. In particular the process comprises transferring the phenolic compounds from the aqueous fluid to an alkaline stripping solution across a non porous, selectively permeable membrane, adjusting the pH of the alkaline stripping solution and separating the resulting phenolic compound rich phase from the acidified stripping solution.
Many phenolic compounds, such as phenol, cresols, nitrophenols, chlorophenols, enter aqueous process streams in chemical processing. These molecules are in many cases toxic. Methods for removing toxic organic molecules from aqueous process streams are well known. Some of these methods use membranes.
Membrane solvent extraction using microporous membranes to provide a phase contacting between aqueous and organic streams is well known. For example Kiani, Bhave and Sirkar Journal of Membrane Science 20 (1984) pp 125-145 report the use of microporous membranes for immobilising solvent interfaces during solvent extraction. Toppings, Micheals and Peretti Journal of Membrane Science 75 (1992) pp 277-292 report using microporous polypropylene fibres to stabilise phase interfaces during extraction of nitrophenol from an aqueous solution into octanol. U.S. Pat. No. 5,512,180 describes a process wherein polypropylene glycol MW 4000 was used to extract nitrophenol in a microporous membrane contactor.
A continuing problem with membrane supported solvent extraction with microporous membranes is the breakthrough of one phase into the other due to pressure imbalances. To overcome this problem, various improvements have been suggested such as using composite membranes comprising a thin layer of non-porous organic-permeable polymer bound to a microporous membrane to avoid phase breakthrough, for example U.S. Pat. No. 4,960,520. However, in all of these processes a solvent phase containing the organic compound is produced which must then be disposed of or treated in some way.
Contacting two aqueous streams with opposite sides of a membrane to effect extraction of organic pollutants from one side to the other is also known in the art. Supported liquid membranes have been applied in this mode. For example U.S. Pat. No. 5,507,949 describes a process wherein the pores of a microporous hydrophobic membrane are filled with a hydrophobic polyamphiphilic oligomeric or polymeric liquid to allow mass transport of various organics across the membranes. In this application the driving force for extraction across the supported liquid membranes may be provided by a stripping solution. The driving force produced by a stripping solution may rely upon conversion of an organic acid to its corresponding salt using a basic stripping solution, or conversion of an organic base to its corresponding salt using an alkaline stripping solution. Biologically active stripping solutions may also be utilised, for example U.S. Pat. No. 4,988,443 to Michaels et al. discloses a method for contacting an aqueous waste stream containing organic toxicants with a nutrient-containing aqueous stream using hollow fibre membranes with water immiscible solvent filled pores. The two streams do not mix but the organic toxicants are transferred from the waste stream across the membrane to the nutrient stream. Micro-organisms growing associated with the outside of the hollow fibres utilise the nutrients and organic toxicants as growth substrates which provides the driving force for continued transport.
In further applications non-porous membranes have been employed to effect extraction of organic molecules from one aqueous stream into another. U.S. Pat. No. 5,552,053 discloses solid polyamphiphilic polymer films used for keeping separate two aqueous phases, one being a waste stream and the other a stripping solution in which the organic pollutant can be concentrated by conversion into an ionised form at controlled pH.
In the above prior art, membranes are substantially rigid and are employed in shell and tube modules, in plate and frame modules, or in spiral wound modules. These modules are designed to generate good mass transfer and fluid distribution around all of the membrane surfaces.
In a few cases, tubular elastomeric non-porous homogeneous membranes for example silicone rubber (cross linked polydimethoxysiloxane) tubes have been disclosed. The tubular elastomeric membranes provide separation by allowing specific chemical species (for example, hydrophobic organic molecules such as benzene, toluene, or their derivatives) to preferentially dissolve in the membrane and permeate across the membrane by diffusion under the influence of a chemical activity driving force. For example, U.S. Pat. No. 5,585,004 to Livingston discloses a system of apparatus and method wherein a waste stream containing toxic organic compounds is fed to the inside of selectively permeable silicone rubber membrane tubes suspended in a bioreactor receptacle filled with a biologically active medium. The toxic organic compounds diffuse across the silicone rubber membrane and into the biologically active medium where they are destroyed by the microbial culture.
Further examples of the use of tubular elastomeric membranes are oxygenation of microbial systems (Cote et al, Journal of Membrane Science 1989 47 p107), and pervaporation (Raghunath and Hwang, Journal of Membrane Science 1992 65 p147). In the field of chemical analysis, silicone rubber membranes have been used to extract organics from aqueous streams prior to analysis (U.S. Pat. Nos. 4,715,217; 4,891,137).
The processing of organic-laden stripping solutions comprising organic acids in dissociated form in an aqueous solution is known with regard to nitrophenolic compounds recovery. For example, various processes are known in the art for disposing of stripping solutions containing nitrophenolic materials. These stripping solutions are generated as a by-product of nitration reactions. U.S. Pat. No. 4,597,875 discloses a process for removing the nitrophenolic materials from an alkaline stripping solution by treating the wastewater with an acid to lower its pH and convert the nitrophenolic compounds to a water insoluble solid material which is separated out of the wastewater and can be disposed of by incineration. U.S. Pat. No. 4,925,565 discloses a process in which the alkaline stripping solution is treated with acid to lower its pH, following which a substantially water insoluble solvent is used to extract the nitrophenolic compounds from the wastewater at elevated temperature. The solvent is recovered by distillation and the residue containing nitrophenolics can be incinerated. In variations on U.S. Pat. No. 4,925,565, the same inventors use differential control of the pH to recover specific nitrophenolic fractions by solvent extraction (U.S. Pat. No. 4,986,917) and precipitation (U.S. Pat. No. 4,986,920). However, the recovery of the nitrophenolic fraction is complicated by the fact that the nitrophenols form solid precipitates upon acidification of alkaline wastewaters containing ionised nitrophenolic compounds at concentrations above the saturation concentration of non-ionised nitrophenolic compounds in water.
In the prior art utilising membranes for organics removal, the temperature of operation with many membranes is limited to between 50-60xc2x0 C., for example when using microporous polypropylene membranes.
The present invention addresses the problems of the prior art.
In one aspect the present invention provides a process for removing and recovering one or more unassociated phenolic compounds dissolved in aqueous fluid, the process comprising the steps of: (a) transferring the unassociated phenolic compound from the aqueous fluid to an alkaline stripping solution, wherein transfer of the unassociated phenolic compound from the aqueous fluid to the alkaline stripping solution occurs across a membrane; wherein the membrane is a non porous, selectively permeable membrane; (b) regulating the volume of alkaline stripping solution employed relative to the volume of aqueous fluid treated so that the total phenolic compound concentration in the alkaline stripping solution, comprising the sum of the dissociated and unassociated phenolic compound concentrations, is above the solubility of the phenolic compound in the acidified stripping solution of step (d); (c) regulating the pH of the alkaline stripping solution in contact with the membrane to a value at least 0.5 pH units above the acidic dissociation constant of the phenolic compound: (d) adjusting the pH of the phenolic compound containing alkaline stripping solution to a value below the acidic dissociation constant of the phenolic compound and (e) separating the resulting phenolic compound rich phase and the acidified stripping solution.
By the term xe2x80x9cselectively permeablexe2x80x9d it is meant a membrane which is permeable to the unassociated phenolic compound and which is impermeable to the dissociated phenolic compound.
By the term xe2x80x9cphenolic compound rich phasexe2x80x9d it is meant a liquid or solid phase which contains more than 40 wt % phenolic compound.
It will be appreciated that the term xe2x80x9cphenolic compoundxe2x80x9d includes any compound of the formula Rxe2x80x94OH wherein R is or comprises an aromatic group.
The present inventors have found that control of the pH in the alkaline stripping solution assists in the reducing of costs and in increasing the membrane lifetime.
In the present invention, phenolic compounds present in an aqueous fluid in unassociated form are recovered by means of membrane extraction across a membrane. The membrane contains at least one non porous, selectively permeable layer. The phenolic compounds pass into an alkaline stripping solution in which the phenolic compounds undergo dissociation. The alkaline stripping solution is then further processed by adjusting the pH downwards until the phenolic compounds re-associate and precipitate out of solution as phenolic compound rich liquids or solids.
A phenolic compound will undergo a dissociation reaction when the pH of the stripping solution is above the pKa of the phenolic compound where pKa is the acidity constant and is defined as follows (see for example xe2x80x9cOrganic Chemistryxe2x80x9d third Edition by T. W. G. Solomns, John Wiley and Sons, p 680):
Rxe2x80x94OH+H2O⇄Rxe2x80x94Oxe2x88x92+H3O+xe2x80x83xe2x80x83(1)
                    pKa        =                              log            10                    ⁡                      (                                                            [                                      R                    ⁢                                          xe2x80x83                                        ⁢                                          O                      -                                                        ]                                ⁢                                  xe2x80x83                                [                                                      H                    3                                    ⁢                                      O                    +                                                  ]                                            [                                  R                  ⁢                                      xe2x80x83                                    ⁢                  OH                                ]                                      )                                              (        2        )            
where R is an aromatic group containing organic structure.
The phenolic compound containing alkaline stripping solution is subsequently neutralised to acid pH and the phenolic compounds return to unassociated form and precipitate out of solution as organic liquids or solids. The organic liquids or solids are separated from the acidified stripping solution. The separated acidified stripping solution may contain saturation levels of unassociated phenolic compounds and may be cycled back to the aqueous fluid to undergo further stripping. In the present invention the extraction and alkaline stripping solution regeneration stages are integrated so that the streams leaving the process are phenolic compound rich organic liquid and treated aqueous waste respectively.
In step (b) the volume of alkaline stripping solution employed relative to the volume of aqueous fluid treated may be regulated so that the total phenolic compound concentration in the alkaline stripping solution, comprising the sum of the dissociated and unassociated phenolic compound concentrations, is not only above the solubility of the phenolic compound in the acidified stripping solution of step (d), but is also above the solubility of the phenolic compound in water. In an alternative the total phenolic compound concentration in the alkaline stripping solution is above the solubility of the phenolic compound in the acidified stripping solution of step (d), but is no greater than the solubility of the phenolic compound in water. The latter alternative is possible because the acidified stripping solution of step (d) may contain salts. When salts are present the solubility of the phenolic compound in the solution is reduced when compared to pure water.
Thus in one aspect the present invention may provide a process for removing and recovering one or more unassociated phenolic compounds dissolved in aqueous fluid the process comprising the steps of (a) transferring the unassociated phenolic compound from the aqueous fluid to an alkaline stripping solution, wherein transfer of the unassociated phenolic compound from the aqueous fluid to the alkaline stripping solution occurs across a membrane; wherein the membrane is a non porous, selectively permeable membrane; (b) regulating the volume of alkaline stripping solution employed relative to the volume of aqueous fluid treated so that the total phenolic compound concentration in the alkaline stripping solution, comprising the sum of the dissociated and unassociated phenolic compound concentrations, is above the solubility of the phenolic compound in water; (c) regulating the pH of the alkaline stripping solution in contact with the membrane to a value at least 0.5 pH units above the acidic dissociation constant of the phenolic compound; (d) adjusting the pH of the phenolic compound containing alkaline stripping solution to a value below the acidic dissociation constant of the phenolic compound and (e) separating the resulting phenolic compound rich phase and the acidified stripping solution.
In a further aspect the present invention provides a process for removing and recovering one or more unassociated phenolic compounds dissolved in aqueous fluid, the process comprising the steps of: (a) transferring the unassociated phenolic compound from the aqueous fluid to an alkaline stripping solution, wherein transfer of the unassociated phenolic compound from the aqueous fluid to the alkaline stripping solution occurs across a membrane; wherein the membrane is a non porous, selectively permeable membrane; (b) regulating the volume of alkaline stripping solution employed relative to the volume of aqueous fluid treated so that the total phenolic compound concentration in the alkaline stripping solution, comprising the sum of the dissociated and unassociated phenolic compound concentrations, is above the solubility of the phenolic compound in the acidified stripping solution of step (d) and no greater than the solubility of the phenolic compound in water: (c) regulating the pH of the alkaline stripping solution in contact with the membrane to a value at least 0.5 pH units above the acidic dissociation constant of the phenolic compound; (d) adjusting the pH of the phenolic compound containing alkaline stripping solution to a value below the acidic dissociation constant of the phenolic compound and (e) separating the resulting phenolic compound rich phase and the acidified stripping solution.
Preferably the aqueous fluid is an aqueous process stream.
Preferably the aqueous fluid is contacted with one side of the membrane and wherein the alkaline stripping solution is contacted with the other side of the membrane. In a more preferred aspect prior to adjusting the pH of the phenolic compound containing alkaline stripping solution, the alkaline stripping solution is removed from contact with the membrane.
Preferably the alkaline stripping solution separated in step (e) is recycled to the aqueous fluid prior to contact with the membrane. In one preferred alternative the alkaline stripping solution separated in step (e) is recycled to the phenolic compound containing alkaline stripping solution prior to removing the alkaline stripping solution from contact with the membrane.
The resulting phenolic compound rich phase of step (e) may be a liquid or a solid.
The membrane of the present invention can be configured in accordance with any of the designs known to those skilled in the art, such as spiral wound, plate and frame, shell and tube, and derivative designs thereof The membranes may be of cylindrical or planar geometry.
For shell and tube designs, the membrane comprises one or more tubular membranes. In this aspect either the aqueous fluid or the alkaline stripping solution is held within the internal volume of the tubular membrane(s) and the other of the aqueous fluid or the alkaline stripping solution is in contact with the external surface of the tubular membrane(s). For spiral wound designs, either the aqueous fluid or the alkaline stripping solution is within the membrane leaves and the other of the aqueous fluid or the alkaline stripping solution is in contact with the external surface of the membrane leaves.
It will appreciated that in an industrial setting preferably the aqueous fluid is held within the internal volume of the tubular membrane(s) and the alkaline stripping solution is in contact with the external surface of the tubular membrane(s), and wherein the tubular membrane(s) and the alkaline stripping solution are operably contained.
In yet further industrial settings preferably the alkaline stripping solution is held within the internal volume of the tubular membrane(s) and the aqueous fluid is in contact with the external surface of the tubular membrane(s), and wherein the tubular membranes and the alkaline stripping solution are operably contained.
The membrane of the present invention is formed from or comprises a material suitable to provide a non-porous, selectively permeable membrane. The membrane may consist of a homogeneous membrane such as a tube or sheet of material, or a composite membrane. The composite membrane may comprise a non-porous, selectively permeable layer and one or more further materials or may comprise a mixture of materials. The non-porous, selectively permeable layer or material prevents direct contact of the aqueous stream with the alkaline stripping solution. This is important. If a direct contact stripping device such as a packed or plate column or microporous membrane contactor is used, the two streams would mix and there would be no resulting separation.
In a preferred aspect the membrane or the non-porous, selectively permeable layer thereof is formed from or comprises a material selected from modified polysiloxane based elastomers including polydimethylsiloxane (PDMS) based elastomers, ethylene-propylene diene (EPDM) based elastomers polynorbomene based elastomers, polyoctenamer based elastomers, polyurethane based elastomers, butadiene and nitrile butadiene rubber based elastomers, natural rubber, butyl rubber based elastomers, polychloroprene (Neoprene) based elastomers, epichlorohydrin elastomers, polyacrylate elastomers, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF) based elastomers, and mixtures thereof
In a preferred aspect the membrane comprises a reinforcing material selected from an external mesh and support. This is particularly advantageous for homogenous tubes or sheets. Such tubes or sheets may be reinforced to increase their burst pressure, for example by overbraiding tubes using fibres of metal or plastic, or by providing a supporting mesh for flat sheets.
When the membrane comprises a non-porous layer and an additional component, the additional component may be a supporting layer. The supporting layer may be a porous support layer. Suitable materials for the open porous support structure are well known to those skilled in the art of membrane processing. Preferably the porous support is formed from or comprises a material selected from polymeric material suitable for fabricating microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes, including polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF) polyethersulfone, and mixtures thereof
Preferably the tubular membranes have a high length to diameter ratios for example the tubular membranes may have internal diameters from 0.5 to 5.0 mm, and/or a wall thicknesses between 0.1 and 1.0 mm and/or a length of from 50 to 500 meters. The length to diameter ratio of the tubular membrane may be from 1xc3x97104 to 1xc3x97106.
High length to diameter ratio such a those given above are considerably longer than the length to diameter ratios of membranes typically applied in prior art membrane extraction processes, and have the advantage that the aqueous fluid entering the membrane tubes passes down a long flow path before emerging from the membrane. Thus it is possible to remove a high percentage of the phenolic compound contaminants in one pass down a single membrane tube, and this reduces the need for extensive manifolding which arises when the aqueous fluid must be passed through several or many membrane modules to achieve the desired degree of removal. This reduction in manifolding results in cost advantages over shorter membrane tubes.
In a further preferred aspect of the present invention a pH control system is used to regulate the flow of alkaline stripping solution which contacts the membrane.
Control of pH in the alkaline stripping solution is important. Upon contact with the membrane the alkaline stripping solution pH will tend to be decreased by the dissociation of the phenolic compound, and it is advantageous for the process efficiency that the pH of the alkaline stripping solution is kept at least 0.5 pH units above the pKa of the phenolic compound. This may be achieved by fixing the flowrate and strength of the alkaline stripping solution so as to ensure that this condition is always met. A higher alkali concentration in the alkaline stripping solution for given volumes or flows of aqueous fluid and alkaline stripping solution will meet this condition better than a lower concentration of alkali. A higher alkali concentration also makes possible a lower alkali flowrate for a given phenolic compound loading in the aqueous fluid, this results in a lower recycle stream flowrate from step (e), and hence a more cost effective system However use of excessive alkali in the alkaline stripping solution will require excess acid in the recovery stage.
Phenolic compounds are known to form two phase mixtures with water, where one phase is a phenolic compound rich phase, and the other phase is a water rich phase. For example xe2x80x9cSolubilities of Organic Compoundsxe2x80x9d Volume II p 373 by A. Seidell, third edition, Van Nostrand Company, New York 1941 provides data showing that at 30xc2x0 C. phenol and water can exist as a phenol rich phase comprising 70 wt % phenol and a water rich phase comprising 91 wt % water.
High ionic strength in the aqueous phase serves to reduce the concentration of water in the phenolic compound rich phase and also reduces the concentration of phenolic compound in the aqueous phase, relative to the levels in a pure waterxe2x80x94phenolic compound system. In the present invention, all other things being equal, the use of higher alkali concentration in the stripping solution, and the use of a higher acid concentration in the acid solution lead to a higher ionic strength in the acidified stripping solution from step (d), and so to a higher percentage of phenol recovered in the phenol rich phase and to a lower concentration of phenol in the acidified stripping solution which is recycled to the process. Hence in one preferred embodiment of the present invention the alkali concentration in the stripping solution and the acid concentration in the acid solution are as high as possible without causing loss of selective permeability of the membrane through chemical attack.
Preferably, the alkali concentration in the stripping solution and the acid concentration in the acid solution are such that acidified stripping solution separated from the phenolic compound rich phase has a salt concentration of greater than 5 wt %, preferably greater than 10 wt %, preferably greater than 20 wt % and preferably greater than 25 wt %.
Preferably the stripping solution in contact with the nonporous membrane is well mixed so that its composition is well mixed throughout the volume operably in contact with the nonporous membrane.
Preferably the pH of the alkaline stripping solution in contact with the non-porous membrane is controlled so that it is substantially the same throughout the alkaline stripping solution in contact with the non-porous membrane separating layer.
Preferably the aqueous fluid contains a phenolic compound selected from phenol, cresols, chlorophenols, dichlorophenols, dimethylphenols, nitrophenols, bromophenols, chlorocresols, benzenediols, benzoquinones, and mixtures thereof
Preferably the alkaline stripping solution comprises a mineral alkali selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof
Preferably the pH of the phenolic compound containing alkaline stripping solution is adjusted in step (d) by the addition of an acid.
Preferably the acid is an aqueous solution of an acid selected from hydrochloric acid, sulphuric acid, nitric acid, phosphoric acid, and mixtures thereof
In a further preferred aspect the aqueous fluid is contacted with one side of a plurality of membranes in series, in parallel or in a combination thereof, and wherein the alkaline stripping solution is contacted with the other side of each of the plurality of membranes.
In further preferred aspect contact between the alkaline stripping solution and molecular oxygen is partially, substantially or completely prevented. This aspect is advantageous because yield of phenolic compounds is improved during the recovery stages (d) and (e) of the process. Without being bound by theory, it is believed that this is due to oxidation reactions of the phenate ion which occur under alkaline conditions in the presence of molecular oxygen (see for example xe2x80x9cRecovery process for phenolic compounds from coal-derived oils by ions of soluble metal saltsxe2x80x9d Ge Y. and Jin, H. FUEL 1996 Volume 75 Number 14 pages 1681-1683). Preferably, exposure of the alkaline stripping solution to molecular oxygen can be limited or prevented by careful construction and operation of the process equipment employed, so that vessels for the stripping solution and/or wastewater are operated full of liquid and with no gaseous headspace. Preferably, exposure of the alkaline stripping solution to molecular oxygen can be limited or prevented by nitrogen sparging of the stripping solution and/or the gas headspace above the stripping solution, and/or by inert gas, preferably nitrogen, sparging of the wastewater and/or the gas headspace above the wastewater. By the term xe2x80x9cinert gasxe2x80x9d it is preferably meant a gas containing oxygen at levels below 1 wt. %.
The process may be performed in a continuous, semi-continuous or discontinuous (batch mode) manner. In the latter aspect the flow of at least one of the aqueous fluid, the alkaline stripping solution, and the alkali solution is discontinuous.
In one aspect the resulting phenolic compound rich phase of step (e) is contacted with an organic solvent and subsequently treated in a further process. In this aspect it may be desirable to contact the phenolic compound containing alkaline stripping solution and/or the separated phenolic compound rich phase with a solvent or solvent mixture in step (e). This may be particularly useful when the separated phenolic compound rich phase is a solid. The solvent introduced may dissolve the solid. This may be further useful when this solid is a product or reactant in a reaction and where the solid and the solvent used to dissolve the solid can be sent to the further process in which the solid material is produced or consumed.
The process of the present invention may be performed in a reactor comprising at least a first zone, a second zone, a third zone, and a fourth zone; wherein each of the zones is discrete from each other zone; wherein the first zone and the second zone are separated by the non porous membrane; wherein the first zone contains the aqueous fluid; wherein the second zone and fourth zone contain the alkaline stripping solution; wherein the third zone contains phenolic compound containing alkaline stripping solution; wherein the third zone and the fourth zone are operably connected to each other; wherein the second zone is operably connected to the fourth zone; and wherein the alkaline stripping solution is circulated between the fourth zone and the second zone such that the alkaline stripping solution is well mixed throughout its volume.
Preferably, the alkaline stripping solution is circulated between the fourth zone and the second zone at a high rate relative to the flow of aqueous fluid. By the term xe2x80x9chigh ratexe2x80x9d it is preferably meant that the volume of alkaline stripping solution contacted with the membrane is greater than the volume of aqueous fluid contacted with the membrane. The ratio of alkaline stripping solution volume to aqueous fluid volume contacted with the membrane may be  greater than 2:1,  greater than 5:1, or  greater than 10:1. A pH control system may be used to regulate the flow of alkaline stripping solution between the fourth zone and the second zone.
The aqueous fluid and/or the alkaline stripping solution of the present invention may be heated before or during contact with the membrane. The aqueous fluid and/or the alkaline stripping solution of the present invention may have a temperature above room temperature (25xc2x0 C.). This may increase the rate of mass transfer across the non-porous membrane. In a further preferred embodiment, the temperature of the aqueous fluid and/or the alkaline stripping solution may be above 60xc2x0 C. In yet a further preferred embodiment, the temperature of the aqueous fluid and/or the alkaline stripping solution may be above 70xc2x0 C.
It is known as for example in xe2x80x9cSolubilities of Organic Compoundsxe2x80x9d Volume II p 373 by A. Seidell, third edition, Van Nostrand Company, New York 1941 that at temperatures above 65xc2x0 C. phenol and water can be totally miscible. In the present invention, temperatures may rise above ambient upon addition of mineral acid to the alkaline stripping solution in step (d), or they may be deliberately raised to increase mass transfer of the phenolic compound in step (a). In one preferred embodiment, the alkaline stripping solution from step (d) is cooled prior to step (e) to effect an improved separation of the phenolic compound rich phase and the acidified stripping solution.
In a further preferred aspect the aqueous fluid contains substantial quantities of dissolved inorganic or organic materials. By the term xe2x80x9csubstantial quantitiesxe2x80x9d it is meant greater than 0.1 wt %. The inorganic materials may include salts, such as sodium chloride, potassium chloride and mixtures thereof The organic materials may include solvents, such as methanol, ethanol, acetone, acetate and mixtures thereof
The phenolic compound in the alkaline stripping solution dissociates according to an equilibrium reaction described by equation (1). Even at high pH, there will be some finite fraction of the phenolic compound present in unassociated form, and the total phenolic compound concentration will be equal to the sum of the concentration of dissociated and the concentration of unassociated phenolic compound. In general, the higher the concentration of total phenolic compound in the alkaline stripping solution at a given pH, the higher will be the concentration of unassociated phenolic compound. This unassociated phenolic compound will act to reduce the driving force for mass transfer of unassociated phenolic compound from the aqueous fluid to the alkaline stripping solution.
This effect will be relatively greater for the aqueous fluid in the section of membrane near the point of exit of the aqueous fluid from the membrane.
Thus in a further preferred embodiment of the present invention, it is desirable to use two well mixed stripping stages in series. In this embodiment, the aqueous fluid first contacts a membrane whose other side is in contact with a well mixed strength 1 alkaline stripping solution in a first stripping stage, and then contacts a second membrane whose other side is in contact with a well mixed strength 2 alkaline stripping solution in a second stage. Strength of an alkaline stripping solution is determined by the strength of the alkali, for example, the mineral alkali, fed to the alkaline stripping solution. In this aspect, the mineral alkali concentration fed to stripping solution 1 is stronger than the mineral alkali concentration fed to stripping solution 2. The aqueous fluid passes from the membrane of stripping stage 1 to the membrane of stripping stage 2. Mineral alkali is fed to the alkaline stripping solution in stripping stage 2, and the resulting strength 2 stripping solution from stage 2 is passed into stage 1 where further mineral alkali is added to increase the strength of the alkaline stripping solution in stage 1 to strength 1. The total phenolic compound concentration in stage 1 is greater than the total phenolic compound concentration in stage 2. The pH may be controlled to be constant in each stripping stage and may be set at different values in stage 1 and stage 2. The use of more than two stages is also envisaged.